The present invention is a building automation system which includes an intrusion alarm and a thermostat in communication with each other. The building automation system thermostat includes means and a method for accumlating an error temperature over time, and creating an alarm signal indicative of an HVAC alarm system fault when the accumlated error exceeds a predetermined limit. This means or method may be located in either the thermostat or the intrusion alarm. The intrusion alarm system, which may be connected to a central monitoring station, can notify both the building owner and the central monitoring station of the HVAC system fault.